The invention relates to hearing aids, and more particularly relates to hearing aids of the type having automatic gain control ("AGC"). In its most immediate sense, the invention relates to hearing aids having both input AGC and output AGC.
Hearing aids commonly have input AGC or output AGC, and at least one currently-available hearing aid having both. Input AGC is used for input compression, so that the response of the hearing aid circuit is properly matched to the patient's hearing impairment. Output AGC is used to prevent the patient from discomfort caused by abruptly loud noises, e.g. by the slam of a door. However, in the hearing aid that has input AGC and output AGC, the circuit is liable to function improperly. This is because this circuit uses two separate and independent AGC circuits. As a result, the circuit uses a relatively high number of components and can malfunction when the tolerances of those components add in the wrong way.
It would be advantageous to provide a hearing aid circuit, and a hearing aid, which would provide a patient with the advantages of both input AGC and output AGC while being simpler and less sensitive to tolerance variations of component values.
One object of the invention is to provide a hearing aid circuit, and a hearing aid, having both input AGC and output AGC while using a relatively simpler circuit.
Another object is, in general, to improve on known hearing aids and hearing aid circuits.
In accordance with the invention, AGC is carried out using a single variable-gain amplifier. This variable-gain amplifier is controlled in response to two signals; one derived from the input (microphone) side of the hearing aid circuit and the other derived from the output (receiver) side. Advantageously, and in accordance with the preferred embodiment, the signals are combined and routed through a single attack/release circuit. As a result, the circuit is greatly simplified and becomes less sensitive to tolerance variations of component values.